156 PHOTOMICROGRAPHY 



advantage that it may be easily dyed to any desired 

 colour in order to produce a series of screens of definite 

 transmissions . 



Sets of Screens. Messrs. Sanger-Shepherd and Messrs. 

 Wratten and Wainwright make screens of dyed gelatine 

 film for photomicrography whose absorption spectra are 

 given, and such a series puts light control on a definite 

 basis and cannot be too strongly recommended. The 

 makers provide a list of the wave lengths transmitted 

 by all of them separately, and by pairs which together 

 transmit approximately monochromatic bands. It is 

 best to have these screens mounted between optical 

 flats, as the film is very easily scratched ; to cement 

 them between ordinary pieces of glass may do well 

 enough for low powers, but the liability to the introduc- 

 tion of errors is then even greater than with ordinary 

 glass screens. They are obtainable in \ and 2" squares. 



To Make Screens of Dyed Gelatine. It is possible to 

 make screens of this type by clearing old lantern slides in 

 Farmer's Reducer and dyeing the film to the desired colour. 

 To preserve the screen a cover-glass is bound over it, as 

 with lantern slides, or two may be bound together face 

 to face to increase the absorption. Better screens can be 

 made by coating thin optical flats with gelatine and 

 then dyeing them, rather than by making use of cleared 

 photographic plates or lantern slides. 



The solution of gelatine is made by soaking one part of 

 best gelatine in ten to thirteen parts of water until well 

 softened ; complete solution is obtained by placing the 

 containing vessel on a water bath, or in a shallow 

 saucepan of boiling water. The solution is filtered whilst 

 hot through glass wool and a little poured on the clean 

 glass. By carefully tilting first to one corner and then 

 to the others an even coating is easily obtained, and any 

 superfluous gelatine run back into the bottle. When the 

 film has set, it is dyed by immersion in the dilute dye 



